This invention relates to electrochemical cells and more particularly to sintered nickel plates for electrochemical cells.
In the nickel-cadmium alkaline cell, porous nickel plates are used to construct both the positive and negative electrodes. The active material for the positive and negative electrodes is contained within the nickel plates. The positive plate contains nickel hydroxide while the negative plate contains cadmium hydroxide. The most efficient structural construction is the sintered nickel plate. This plate is designed for a high surface area to plate volume ratio. To form the plates, fine nickel powder (carbonyl nickel) on a wire screen is placed in a mold and sintered at elevated temperatures (700.degree.-1000.degree. C.) in a reducing atmosphere. The result is a plate (plaque) about 0.030-0.060 inches thick and 70-85% void (porosity).
Sintered nickel plates are lighter and have greater effective areas than do solid nickel plates. However, sintered nickel plates are limited by several factors to certain minimum weights. For example, very high porosities produce a structurally weak plaque which cannot endure the electrical stresses involved in impregnation and cycling. On the other hand if large cavities (pockets) are designed into the plate to hold significant amounts of active material, the problem becomes one of maintaining sufficient electrical conductivity.
In U.S. Pat. No. 3,476,604 entitled "Method of Making an Electrode Grid," issued to Peter Faber on Nov. 4, 1969, an electrode grid is formed by sintering a web of nickel-boron coated carbonaceous fibers. In that method, a fabric or felt of cellulose fibers was first charred to produce a web of carbonaceous filaments. These carbonaceous filaments were then at least partially graphiticized. Next, an aqueous solution of a nickel salt in the presence of a reducing boron compound was used to deposit a nickel-boron coating on the carbonaceous filaments. Finally, the web of nickel-borom coated carbonaceous filaments was sintered to form the nickel electrode grid.
There are several disadvantages to this prior art approach. First, if carbon or graphite fibers formed by charring materials such as rayon or cellulose fibers are used to produce the sintered nickel electrode grids, the grids swell upon cycling in an alkaline (e.g., KOH) electrolyte. Additionally, carbon or graphite fibers formed in this manner are expensive. Finally, the high purity reducing boron compounds required in the Faber process are very expensive. The less expensive phosphorous reducing compounds have not been tried in the prior art because it was thought that phosphorus co-deposited with the nickel on the fibers would prevent the proper operation of the nickel electrode. In other words, although inert boron does not interfere with the electrochemical reactions of the nickel electrode, reactive phosphorous was expected to interfere and make the cell impractical.